So, why is it recommended to apply the product to dry hair for balayage/ombre and regrowth application?

Because these are localised stripping applications, and you do not want the formula to start spreading throughout the hair when just the regrowth or specific balayage threads are being targeted. In addition, when the formula is applied to dry hair it will be slightly more potent, so balayage threads and regrowth sections will lighten more intensely.